Hey, I live in Czech Republic too. I've been homebrewing for almost two years now. Here's a list of sites in CZ to get ingredients and equipment. You're right, there are no physical shops in Prague though....
https://eshop.sladovna-kounice.cz
https://www.pivoteka.cz
https://www.vyrobtesipivo.cz
http://chmely.cz
http://hopproducts.cz/eshop/?fref=gc&dti=87801789392
https://www.homebrewshop.cz
https://pivoteka.beergeek.cz/homebrewing/
It sounds to me that you're set up to start with extract. Although apparently it's easier go that route at the beginning I wouldn't necessarily recommend it. There are two reasons:
1) Most really good tutorials online are all grain tutorials. The one I really liked was
Beer And BBQ by Larry you can find it on YouTube.
2) The price and quality of grains in this country are amazing and the extract kits and syrups are not really worth your time and money. The Kounice Sladovna (first link) arguably has some of the best grain around and it's dirt cheap. We're talking around 30 Kč per kilogram. For those keeping score back in the states that's about a $1.25 for almost 2.5 pounds of malt.
I'm not sure if you're a native or foreigner but if you can't read/czech you can use Google Translate on those pages and it'll lead you in the right direction.
Na Zdraví!